Pre-1978 no mark. There are no copyright marks on the ad. Copyrights for Saturday Evening Post would not apply to the advertisements in it. RKO, the owner of the ad, did not mark their ads as copyright.
Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.